Conventionally, a freely flowing abrasive slurry is fed to the lapping disc of a lapping machine or the like. The abrasive slurry is usually circulated by means of a pump a delivered to the lapping disc through flow-restrictive discharge tubes or conduit means. Heretofore, it has been extremely difficult to maintain a constant discharge rate of abrasive slurry because of variations in pressure and flow of the pump, and it has also been difficult to adjust conveniently the discharge rate for different operating conditions, because of the perverse nature of various slurries.